1. Field of the Invention
This invention relates generally to diverter and blowout control systems for drilling rigs. In particular, the invention relates to diverter and blowout preventer systems and methods for use with bottom supported offshore drilling rigs.
2. Description of the Prior Art
When drilling a well from a bottom supported offshore rig, it is desirable that a diverter be provided during shallow hole drilling through the drive pipe. Such a diverter is typically provided below a rig floor between the casing and the rotary table of the drilling rig for safely venting or controlling unbalanced well bore pressure which may produce an upward flow of drilling fluid in a conduit. Such an occurrence, called a "kick", typically a pressurized gas accumulation in the fluid of the conduit, is often encountered in top hole drilling making a fluid flow controller essential before high pressure blowout preventers are connected to the drilling system.
U.S. Pat. Nos. 4,456,062, 4,456,063, 4,502,534 and 4,444,401, assigned to the same assignee as the assignee of the present application, disclose a "failsafe" diverting system for a floating drilling rig. The disclosed system is "failsafe" in that when a "kick" occurs during shallow hole drilling of a top hole well before a blowout preventer stack has been provided, the "kick" can not be accidentally confined by the flow diverter apparatus to build pressure and explode, even if controls are misconnected or malfunctioning. The above mentioned patents disclose a diverter or "fluid flow controller" adapted for positioning within a housing permanently secured to the floating drilling rig floor below the rotary table.
Diverters may also be provided for bottom supported offshore drilling rigs. Such diverters, because of their capability of sealing the annular space between the drilling conduit or riser and the drill pipe, may also be used as a low pressure blowout preventer. Such a diverter/BOP system is disclosed in U.S. Pat. No. 4,524,832 to Roche et al. and is assigned to the same assignee as that of this application. Such application is incorporated herein for all purposes.
When a drilling rig operator uses such a diverter/BOP system, he must be assured of the mode that the system is in. If the operator believes the system to be in a diverter mode, but yet the system is in a BOP mode, a kick of pressure greater than the pressure rating of the system could result in catastrophic damage to the drilling rig and its personnel. If the operator believes the system to be in the BOP mode because sufficient casing has been set, but yet the system is actually in the diverter mode, the well cannot be "controlled" in the sense of killing the kick with drilling mud with the blowout preventer closed and circulating it out via a choke line to a choke manifold. Rather, the kick is diverted via the vent line system and may flow until the shallow gas is dissipated. It is necessary therefore to provide systems which can assure the drilling crew and the drilling superintendent that the diverter/BOP system described above is in a "for sure" mode, either as a diverter or as a BOP.